1. Field of the Invention
The present invention relates to a light-emitting device, a flexible light-emitting device and a method for manufacturing the light-emitting device and the flexible light-emitting device. Further, the present invention relates to an electronic device on which the light-emitting device or the flexible light-emitting device is mounted.
2. Description of the Related Art
In recent years, technological development has been remarkably made in the field of displays. In particular, the needs of the market have stimulated tremendous progress in the technology directed to increase in definition of displays and thinning of displays.
In the next phase of this field, focus is placed on commercialization of a flexible display, and a variety of proposals have been made on manufacturing the flexible display (for example, Patent Document 1). A light-emitting device using a flexible substrate can be highly lightweight compared to the case of using a glass substrate or the like.
However, in commercialization of such a flexible display, the biggest problem is its short lifetime.
The lifetime of the flexible display is short because, for a substrate which should support a light-emitting element and protect the element from moisture, oxygen, or the like of the surroundings, a glass substrate that is not flexible cannot be used, and instead, a plastic substrate which has flexibility but high water permeability and low heat resistance has to be used. Since the heat resistance of the plastic substrate is low, a protective film with high quality which needs a high-temperature process cannot be formed, and moisture entering through the plastic substrate has a great influence on the lifetime of the light-emitting element, furthermore, the light-emitting device. In Non-Patent Document 1, for example, an example in which a light-emitting element is formed over a substrate including polyethersulfone (PES) as a base and is sealed with an aluminum film to form a flexible light-emitting device is introduced; however, its lifetime is about 230 hours and the light-emitting device is miles away from commercialization.
Although a metal thin film such as an aluminum film has both flexibility and low water permeability, it does not transmit visible light therethrough with a normal thickness. Thus, in the light-emitting device, a metal thin film is used for only one of a pair of substrates by which a light-emitting element is sandwiched.